Closed Mold RTM Process Mold Material Selection and Production

 

RTM (Resin Transfer Molding) is a process method that injects resin into a closed mold to infiltrate the reinforcing material and solidify it. It is suitable for the molding of multi-variety, medium-volume, high-quality advanced composite materials. This advanced process has many advantages. It can use a variety of fiber-reinforced materials and resin systems and has an excellent product surface. It is suitable for manufacturing high-quality complex-shaped products, with high fiber content, low volatile components during molding, less environmental pollution, strong adaptability to production automation, low investment, and high production efficiency.

 

Therefore, RTM technology has been widely used in the automotive industry, aerospace, defense industry, mechanical equipment, and electronic products. The primary factor that determines RTM products is the mold. Since RTM molds generally use the male and female mold matching method, finding ways to improve the surface quality and dimensional accuracy of the male and female molds has become a key factor in determining product quality.

Material Selection

 

Material selection is a key factor in mold quality. Material selection is based on the characteristics of the RTM molding process.

 

(1) Gel coat layer: RTM molding releases a lot of heat. A 4mm thick product can generally release more than 120°C. This requires the gel coat resin to have heat shock resistance, gloss and heat resistance. This process uses vinyl mold gel coat, which has a heat deformation temperature of 160°C-172°C and good mechanical properties.

 

(2) Surface layer: Mainly considering heat resistance and crack resistance, 30g/㎡ surface tissue and 300g/㎡ alkali-free chopped strand mat are used as reinforcement materials, and bisphenol A epoxy vinyl ester resin is used as resin. This resin has good continuous high temperature resistance and low shrinkage.

 

(3) Reinforcement layer: Emphasis on strength and shrinkage. The selected reinforcement materials are 0.4 alkali-free cloth and 300g/㎡ alkali-free chopped strand mat as reinforcement materials, and zero shrinkage resin is used as the matrix material.

(4) Reinforcement layer: Increase the overall rigidity of the mold to facilitate mold opening and closing operations, and use steel frame reinforcement.

 

 

Production of Original Mold

 

Many original molds of FRP molds have been made of gypsum, wood, cement, paraffin, etc. as base materials by hand-made methods. The surface of the master mold made with these materials and production processes is difficult to reach Class A surface, the dimensional accuracy cannot be controlled, the production process is complicated, the cycle is long, defects are easy to occur, and the flatness is poor. It is only suitable for FRP molds with low precision requirements and low surface quality requirements. If the above method is used, the production requirements of RTM models cannot be met at all.

 

In order to meet the requirements of RTM models, block-shaped machinable resin can be used as raw material for CNC machining. Block-shaped machinable resin is commonly known as wood substitute. Epoxy resin, ABS plastic, glass beads, aluminum dioxide, and carboxymethyl fiber are mixed and stirred evenly. After dibutyl ester and other ingredients are mixed in a certain proportion, the mixture is heated to dissolve into a paste slurry. After vacuum defoaming treatment, it enters the mold for molding. The molded solid is heated to form a solid. Wood substitute has the advantages of good thermoplasticity, strong stability, and good mechanical processing performance.

 

When making a model with wood substitute as the material, first of all, according to the design requirements of the product, use 3D drawing software such as Pro/e, UG, CATIA to draw a 3D model of the product. When designing the 3D digital model of the product, in order to ensure the accuracy of the product, a certain margin should be left according to the shrinkage rate of the selected resin. Then the original mold is processed by CNC machining. The original mold is generally processed into a real sample of the product, and the product size and thickness must meet the design requirements. In this way, the step of making the mold cavity is omitted during production, which greatly improves the size and thickness accuracy of the product.

 

Mold Making

 

Material selection and original mold making provide the premise for the successful making of an RTM mold. However, the control of the production process cannot be ignored. The original mold is processed by CNC, and the dimensional accuracy meets the requirements, but the knife marks of the processing still have a great impact on the mold surface. It needs to be finely polished and polished to make the model meet the A-level surface requirements.

 

First, start the production of the first mold gel coat layer. In order to obtain a good surface effect, the gel coat layer needs to be made by spraying. When spraying the gel coat, the general amount of gel coat is 1000g/㎡, which is sprayed in 3 times. The amount of gel coat for the first layer is 200g/㎡, and the color is different from the next two times. The amount of gel coat for the next two times is 400g/㎡.

 

The airflow of the gel coat spray gun should be uniform, and the glue liquid should be in a mist form and evenly sprayed on the surface of the model. After the last spray of the glue liquid gels, the surface layer is laid.

 

The surface layer is made of a layer of 30g/㎡ surface veil and 4 layers of 300g/㎡ chopped mat. The surface mat should be laid flat without overlap and wrinkles. Be especially careful at the edges and corners, and use the corner of the brush to fully press and drive away bubbles.

 

After the surface layer is cured, it is necessary to pick out bubbles, puncture the bubbles, then sand and remove burrs, and wipe with acetone, and then you can paste the bottom. The 4-layer 300g/㎡ chopped strand mat group is completed in 2 pastes.

Before each paste, it is necessary to confirm that the previous glue is completely cured and the bubble picking work is completed. The paste of the reinforcement layer can be made by alternating cloth and mat, and 4 layers are cured once.

The paste must be laid flat, the joints should be staggered, and do not overlap at the edges and corners. The amount of resin glue in each layer must be strictly controlled to fully wet the fiber but not too much.

After the paste reaches the designed thickness, the reinforcement steel frame can be pasted. There should be no gap between the steel frame and the mold. It is best to make it on site. The steel frame needs to be treated to eliminate welding stress before pasting.

After the first mold is cured for 24 hours, it can be demoulded. Be careful not to let the original mold fall off the mold when demoulding. If you find that the mold is separated, you can use the vacuum method to re-fit the original mold with the first mold.

After simple trimming, you can start making the next mold after placing the injection port and the glue outlet at the designed position.

After making the second mold, put the entire mold in an oven and gradually heat it from room temperature to 75℃, then keep it warm for 2 hours, stop heating, and gradually cool it to room temperature. After welding the guide, you can proceed with demoulding.

 

Post-processing of Mold

 

After demolding, remove excess flash and blow away small particles in the mold with compressed air.

 

Polishing: Soak 400# water sandpaper in water, wrap it on a smooth polishing board, and polish it back and forth repeatedly. Until the first layer of gel coat is completely sanded off; wash the mold with clean water; use 600# water sandpaper to polish according to the above method until there are no obvious scratches of uneven thickness on the mold surface; wash the mold, use 800# water sandpaper, and polish according to the above method until there are no obvious scratches on the mold surface and it feels smooth and delicate; wash with clean water and finally use 1200# water sandpaper to polish it repeatedly according to the above method.

Polishing: Apply 1000# polishing paste evenly on the mold surface. After 2-3 minutes, use a polishing machine with a cloth wheel to polish repeatedly back and forth until the mold surface is mirror-like and there is no residual sanding surface (generally, polishing is required twice); after washing with clean water, apply 2000# polishing paste evenly on the mold surface. After 2-3 minutes, use a polishing machine with a wool wheel to polish section by section until the mold surface is highly clear. After polishing, you can wash off the polishing paste with detergent water and then wash with clean water and dry.

 

Sealing agent: After the mold surface is completely dry, dip the sealing agent with a clean cloth and apply it evenly on the mold surface. Apply it again after an interval of 20-30 minutes.

 

Release agent: Use a high-efficiency liquid release agent and soak a clean cotton gauze with the release agent, but do not drip it. Wipe a layer of smooth coating, wet it, apply it all over, wait for 20 to 30 seconds, and then wipe off the excess release agent with another piece of cotton cloth, but don’t use too much force, just press the mold with the weight of your palm. Repeat this operation 6 times, each time for 20 to 30 minutes.

 

From the entire production process of the RTM mold, it can be seen that the original mold design and production lay the foundation for the production of the entire mold. Due to the adoption of new prototype production technology, the production of prototypes has the advantages of fast, high precision, simple process, easy operation, and no mold cavity production, which not only shortens the construction period, but also meets the development trend of modern enterprises to quickly respond to market demand. Coupled with the control of material selection and mold production, the application of RTM molding process will become more and more extensive.